Aerostats, or moored balloons, are finding increasing use in applications as diverse as surveillance, weather monitoring and renewable energy. Their inherent reliability, low cost and ability to loiter on station for long durations with minimal maintenance or fuel use provide a unique combination of capabilities unmatched by heavier-than-air flight vehicles or satellites. However, aerostats suffer from high downtime and are susceptible to damage in adverse wind conditions. In addition, a full time ground crew is typically required to control the aerostat during launch and landing operations, which significantly increases the operating cost of aerostat systems.
A typical aerostat system consists of an envelope filled with a lighter-than-air (LTA) gas to provide buoyant lift; a base station consisting of a rotating platform with mounting tower and a winch or actuator; and a single main tether connecting the envelope and base station winch. A functional payload is often suspended within or below the envelope.
In flight configuration a specified length of tether is paid out and the buoyant force of the contained LTA gas lifts the envelope and payload. Fins on the envelope may be used to ensure the envelope passively orients into the wind, but the aerostat is otherwise constrained only by the tether. In high winds, the drag force on the aerostat blows the aerostat down wind and reduces the flight altitude. Typical aerostats in flight configuration are susceptible to down drafts, in which a downward vertical component of wind speed results in a rapid loss of altitude and slack in the tether. When the down draft passes and the buoyant force regains dominance the aerostat quickly rises until the tether snaps taught. This shock load is a leading cause of damage to the tether and aerostat envelope, and for this reason, aerostats are often grounded whenever downdraft conditions might occur.
During docking, the winch or actuator will typically reel in the main tether, drawing the aerostat in toward the base station. When the aerostat is near enough, ground crew feed auxiliary tethers into auxiliary winches on the base station. There is typically at least one auxiliary nose tether which is fed in through the mounting tower on the base station platform, which, in combination with the main tether and other auxiliary tethers, allows the aerostat to pivot with the base station platform about a designated axis. Ground crews must be highly skilled because the aerostat can become highly unstable if the auxiliary tethers are improperly secured, resulting in damage to the aerostat, payload and/or base station.
There is currently a strong push towards increasing the safe flight regime of aerostat systems in order to minimize the down time due to weather. In addition, eliminating all or part of the flight crew, whose primary responsibilities include overseeing launch and landing operations, can significantly reduce the cost of operating aerostats. The present invention describes an aerostat system that realizes both an increased stable flight regime and reduced ground crew requirements.